The present invention relates generally to digital data receivers and particularly concerns a digital television signal receiver.
A number of systems have recently been proposed for transmitting and receiving television signals in a digital form. The television signal may comprise, for example, a compressed wideband HDTV signal or one or more compressed NTSC signals. The two most widely promoted modulation techniques being considered for effecting such transmissions are quadrature amplitude modulation (QAM) and vestigial sideband modulation (VSB). U.S. Pat. No. 5,087,975 discloses a VSB system for transmitting a television signal in the form of successive M-level symbols over a standard 6 MHz television channel, with a relatively small (low level) pilot at the lower edge of the channel. While the number of levels M (i.e., the VSB mode) characterizing the symbols may vary, the symbol rate is preferably fixed, such as at a rate of 684 H (about 10.76 Megasymbols/sec.), where H is the NTSC horizontal scanning frequency. The number of symbol levels used in any particular situation is largely a function of the signal to noise (S/N) ratio characterizing the transmission medium, a smaller number of symbol levels being used in situations where the S/N ratio is low. It is believed that a system accommodating symbol levels of 24, 16, 8, 4 and 2 provides adequate flexibility to satisfy most conditions. It will be appreciated that lower values of M can provide improved S/N ratio performance at the expense of reduced transmission bit rate. For example, assuming a rate of 10.76 Megasymbols/sec., a 2-level VSB signal (1 bit per symbol) provides a transmission bit rate of 10.76 Megabits/sec., a 4-level VSB signal (2 bits per symbol) provides a transmission bit rate of 21.52 Megabits/sec., and so on.
Proper operation of a digital television receiver requires the received carrier signal to be acquired relatively rapidly and the gain of the RF and IF sections to be appropriately adjusted. Carrier acquisition in QAM receivers is relatively difficult because of the absence of any sort of pilot. While the use of a pilot in the above-mentioned VSB system greatly facilitates carrier acquisition, some difficulties may nevertheless be encountered due to the relatively low level of the pilot and the limited pull-in range of synchronous demodulators used in VSB receivers. One aspect of the invention enhances pull in of the FPLL and another deals with an improved AGC system. Also, the frequency and phase locked loop (FPLL) in the synchronous demodulator is bi-phase stable. Consequently, the phase of the output data may be inverted and therefore need to be phase reversed. Further, the lock-up characteristics of the FPLL are determined by the characteristics of the AFC filter and in another aspect of the invention, an improvement in such characteristics is provided. The invention also includes an arrangement for optimizing conversion of the analog signal to digital format when a DC offset is used to generate the pilot.